<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
    ul li {
      float: left;
      list-style: none;
      width: 150px;
      height: 200px;
      border: 1px solid slateblue;
      margin-left: -1px;
    }

    ul li:hover {
      /* 方法一 */
      position: relative;
      border: 1px solid salmon;

      /* 方法二 */
      z-index: 1;
      border: 1px solid salmon;
    }
  </style>
</head>
<body>
  <ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
  </ul>
</body>
</html>